1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube provided with a convergence device regulated to prevent convergence error caused by an external magnetic field or the like.
2. Description of the Related Art
In recent years, there has been a growing demand on the market for a color cathode ray tube which allows a user himself to correct convergence error after the color cathode ray tube is shipped, caused by an external magnetic field under working conditions.
Heretofore, correction coils mounted on the rear end of a deflection yoke disposed in front of a CPM (Convergence Purity Magnet) have been used as the major means for correcting convergence slipping of horizontal lines.
In this regard, a color cathode ray tube with a convergence device of the structure shown in FIG. 1 has been disclosed in Japanese Patent Laid-open Publication No. 185812/96.
Color cathode ray tube unit 101 has convergence device 106 comprising deflection yoke 103 mounted on neck portion 102, correction coil 105 wound between CPM 104, and control circuit 107.
Neck portion 102 of a color cathode ray tube of this kind generally has the structure shown in FIG. 2. That is, in the inside of neck portion 102, there is provided the triode portion comprising cathode 117 for emitting electron beams R, G, B, heater 118 for heating cathode 117, G1 electrode 110, G2 electrode 111, and an MFL (Main Focus Lens) comprising a plurality of electrodes including G3 electrode 112 and G4 electrode 113. The position for adjusting the static convergence of electron beam 115 caused by the MFL is shown as MFL position 114. Further, on the outer circumference of neck portion 102, correction coil 105 is disposed in the vicinity of MFL position 114.
However, since a cathode ray tube unit of this structure is arranged so that it acts in the vicinity of the MFL to bend side beams R and B of the electron beams R, G, B, landing transition or raster rotation are caused concurrently with the convergence correction activity.
Landing transition and raster rotation are generated when a convergence correction current is supplied from control circuit 107 to correction coil 105 disposed between deflection yoke 103 and CPM 104 due to changes in orbits of electron beams caused by the magnetic field produced by correction coil 105 which affects the area up to near the deflection point of deflection yoke 103.
Further, the amount of correction of this color cathode ray tube unit applied to correct the convergence error caused by an external magnetic field is rather insufficient. In order to meet the necessary and sufficient amount of correction, there have been no means other than augmenting the number of turns of correction coil 105 or increasing the control current supplied from control circuit 107, both violating the policy for saving power. In other words, when the distance between correction coil 105 and phosphor screen 116 is short, it is required to augment the magnetic force of the correction coil to the side beam for correcting convergence error on phosphor screen 116, and consequently power supply from control circuit 107 to correction coil 105 is inevitably increased.